Ziegler-Natta catalysts are a mainstay for polyolefin manufacture, but single-site (metallocene and non-metallocene) catalysts represent the industry's future. These catalysts are often more reactive than Ziegler-Natta catalysts can can give polymers with improved physical properties.
Transition metal complexes that incorporate “indenoindolyl” ligands are known (see, e.g., U.S. Pat. Nos. 6,232,260 and 6,451,724). In many of the known complexes, an indenoindolyl group is bridged to another group, which may be a second indenoindolyl group. Some of the known bridged indenoindolyl complexes have constrained geometry or “open architecture” (see, e.g., U.S. Pat. No. 6,838,410). Indenoindolyl ligands are versatile because a wide variety of indanone and arylhydrazine precursors can be used to produce indenoindoles. Thus, substituent effects can be exploited and catalyst structure can be altered to produce improved polyolefins.
Non-bridged indenoindolyl complexes generally provide good operability. They usually give polyolefins having a desirably high bulk density, narrow particle size distributions, and a low or undetectable level of polyolefin “chunks,” i.e., agglomerated polymer particles. High bulk density and good resin morphology are crucial for commercial manufacturing processes because they impact cost, productivity, and process viability.
Unfortunately, non-bridged complexes are not ideal for achieving high polyolefin molecular weight or efficient comonomer incorporation. In our experience, introducing bridging into indenoindolyl complexes allows one to boost polyolefin molecular weight and to achieve desirable product density targets. However, bridging can adversely impact operability. In particular, the bridged complexes can give polyolefins with agglomerated polyolefin chunks, broad particle size distributions, and low bulk densities.
Organozinc compounds are known chain-transfer agents. Thus, when an olefin is polymerized in the presence of an organozinc compound, the expected result is a reduction in polymer molecular weight. In copending application Ser. No. 10/614,615, filed Jul. 7, 2003, now allowed, we explained the benefits of pretreating silica supports with an organozinc compound. In particular, we showed that indenoindolyl transition metal complexes supported on organozinc-treated silicas are highly active and give polyolefins with unexpectedly high molecular weight. Less than clear is the impact of using an organozinc compound in the polymerization process rather than as a component of an indenoindolyl transition metal catalyst system. Will it behave as a chain-transfer agent and reduce polymer molecular weight, will it increase molecular weight (as it did when it was part of the catalyst system), or are there other possible outcomes?
Ideally, polyolefin manufacturers could make high-molecular-weight resins using bridged indenoindolyl complexes while enjoying the operability benefits normally associated only with non-bridged varieties of these complexes. A desirable process would enable the preparation of high-molecular-weight polyolefins having a low or undetectable level of polyolefin chunks, narrow particle size distributions, and high bulk densities.